As antenna matching technology on the aforementioned antenna matching circuit and the like, for example, the following are known.
Patent Document 1 Japanese Patent Application Laid-open No. Hei 5-327331.
A matching means described in Patent Document 1 uses a parallel resonant circuit composed of one L and one C, and it is known as a susceptance compensation method (See paragraph numbers 0006, 0007, FIGS. 2, 4, 6). Patent Document 2 Japanese Patent Application Laid-open No. 2000-286615
A matching means described in Patent Document 2 uses a π-type tuning circuit, and this matching means is also commonly known as an antenna matching means (See a paragraph number 0022, FIG. 4).
Each of the aforementioned documents discloses the technology for integrally fabricating the aforementioned matching means inside a chip antenna, and provides means for integrally matching a monopole, a dipole, or an inverted F type antenna in a chip.
However, according to the aforementioned matching means, a matching property in a chip antenna alone is improved, but performance demanded by a communication device sometimes cannot be covered since the band is not sufficiently widened. Namely, according to the aforementioned Patent Document 1, as shown in FIG. 8 in this document, only a band of 40 MHz with 820 MHz as a center frequency is covered. Concerning a frequency band around 5 GHz in recent years, bands in the vicinity of 4.9 GHz, the vicinity of 5.2 GHz, the vicinity of 5.4 GHz, and the vicinity of 5.8 GHz are used in Japan, the United States, and Europe, respectively. The band of use of the 5 GHz band at the present time is a band of approximately 1 GHz ranging from 4.9 GHz to 5.925 GHz in all countries. If a VSWR of 2 or less is a usable range, as far as the inventors know, there are few antennas relatively small in size and capable of covering the 1-GHz worldwide band at the VWSR of 2 or less.
Hence, the inventors fabricate a chip antenna of the same type as the chip antenna disclosed in Patent Document 2 and conduct an experiment. The chip antenna has dimensions of 8.0×3.0×1.0 mm. As shown in FIG. 21a, a circuit used in the experiment includes an antenna substrate 121, a chip antenna 123 at one end of the antenna substrate, and a GND portion 125 disposed adjacent to the chip antenna 123. The resonant frequency of the chip antenna 123 is set to the 2.5 GHz band, and a length L of the GND portion 125 is set at a length equal to a quarter of a resonant wavelength. As shown by a graphic chart in FIG. 21b, a region with the VSWR of 2 or less (usable region) ranges over 76 MHz from 2410 MHz to 2486 MHz.
Next, if the length L of the GND portion 125 is set at a third of that of the GND portion 125 shown in FIG. 21A as shown in FIG. 22a, the usable region is shifted to higher frequencies as shown in FIG. 22b, whereby the antenna cannot be used due to mismatching in the 2.5 GHz band in which the use of the antenna is required.
Hence, as shown in FIG. 23a, matching is attempted by adding a π-type matching circuit 127 composed of L, C1, and C2 similar to a matching circuit disclosed in Document 2. As a result, it is possible to produce resonance at a desired frequency band, but as shown in FIG. 23b, the region with the VSWR of 2 or less can be secured only in a range of 32 MHz.
As is evident from the aforementioned experiment, when the length of the GND portion is made shorter than a quarter of a wavelength to be used in order to reduce the size, a matching circuit is needed to perform matching at a frequency to be used, but if the r-type circuit is used as this matching circuit, the usable region with the VSWR of 2 or less is secured only in a range of 32 MHz. In order to improve these circumstances, an object of the present invention is to provide antenna matching technology effective in achieving a wider band.